The invention relates to a device for controlled batch feeding of a fluidizible particulate material to a reaction chamber from a silo featuring at least one outlet opening, in particular alumina from a day's storage silo to a break in the crust in a fused salt electrolytic cell for producing aluminum, and relates also to a process for operating the said device.
In the production of aluminum by the fused salt electrolytic reduction of aluminum oxide the latter is dissolved in a fluoride melt made up for the most part of cryolite. The cathodically precipitated aluminum collects under the fluoride melt on the carbon floor of the cell, the surface of the liquid aluminum forming the actual cathode. Dipping into the melt from above are anodes which in conventional processes are made of amorphous carbon. At the carbon anode oxygen is formed as a result of the electrolytic decomposition of the aluminum oxide; this oxygen combines with the carbon of the anode to form CO.sub.2 and CO. The electrolytic process takes place in a temperature range of about 940.degree.-970.degree. C.
During the course of the electrolytic process, the electrolyte becomes depleted in aluminum oxide. At a lower concentration of 1-2 wt.% aluminum oxide in the electrolyte the anode effect occurs whereby the voltage increases from 4-5 V to 30 V and more. Then at the latest the crust of solid electrolyte must be broken open and the aluminum oxide concentration raised by addition of alumina.
Under normal operating conditions the cell is serviced this way at regular intervals even when no anode effect occurs. In addition each time the anode effect occurs the crust must be broken open and the concentration of aluminum oxide raised by the addition of alumina, which corresponds to a servicing of the cell.
For many years now this servicing of the cell has been carried out by breaking open the crust between the anodes and the side of the cell and adding the alumina there. This practice has, however, met with increasing disapproval due to the pollution of the air in the pot room and the surrounding atmosphere. With hooded electrolytic cells maximum capture of the fumes from the cell is possible only if this servicing of the cell is automated. After breaking open the crust, the alumina is fed either locally and continuously according to the point feeder method or discontinuously over the whole longitudinal or transverse axes of the cell.
The known storage bunkers or silos mounted on the reduction cells are generally in the form of funnels or containers with funnel-shaped or conical outlet parts at the bottom. The contents of the silo on the cell usually meet one to two days requirements for the cell and can therefore be called day-storage silos.
The supply of alumina from the silo to the break in the crust on the reduction pot is made via conventional devices by opening a flap which is tilted to feed the cell, or via other systems using screw feed arrangements, pistons or the like.
These known batch feeding devices have the disadvantage that mechanically movable parts have to be built in the anodic part of the reduction cell. Consequently they are subjected to the adverse effects of the cell atmosphere, heat and dust, which requires a more or less thorough maintenance for their satisfactory operation.